1. Field of the Invention
The present invention relates generally to a refrigerated automatic fluid sampling apparatus having a top-mounted compressor assembly to enhance thermal efficiency and corrosion resistance, a thermal control system which optimizes compressor operation, and a sampling control system which uses a computer-simulated statistical model to deliver desired sample volumes with precision.
More particularly, the invention relates to a microprocessor-controlled refrigerated sampling apparatus with a weather-resistant external housing having a thermally-controlled sample compartment provided in a lower portion thereof, and a controller unit and compressor assembly mounted in an upper portion thereof. The controller unit includes a computer control means connected with a sample pumping system and a thermal control system for the compressor assembly. The thermal control system exercises the compressor assembly during periods of non-use, prevents short cycling of the compressor, and employs a unique sensor for accurately monitoring sample temperatures so as to minimize compressor starts. The sampling control system uses a statistical model to simulate the real-life pumping system and operating characteristics, permitting accurate delivery of a preselected sample volume.
2. Description of Relevant Art
To preserve sample integrity during the wastewater sample collection process, it is often necessary to refrigerate collected samples to prevent chemical and biological degradation due to ambient temperature extremes. Under current Environmental Protection Agency ("EPA") regulations, wastewater samples are required to be maintained at 4.degree. C.
In known refrigerated sampler devices, the compressor is mounted at the bottom of the unit, near the floor and below the sample compartment. This known arrangement has a number of disadvantages. The bottom mounted compressor is not only inconveniently located for servicing, the heat generated by the compressor rises up and around the sample compartment, forcing the system to work harder. Further, bottom mounted compressors are exposed to the effects of highly corrosive heavier-than-air gases, such as chlorine and hydrogen sulfide, by-products of wastewater which hug the floor. Because the floors and walkways within waste treatment plants are regularly hosed down, bottom mounted compressors and associated refrigeration components are subjected to splatter and spray which accelerates the corrosion process.
The refrigerated sampling apparatus according to the present invention overcomes the problems associated with known bottom-mounted compressor arrangements by mounting the compressor assembly at the top of the unit, above the sample compartment. The top mounted compressor assembly of the invention not only lends itself to convenient servicing, it permits heat generated by the compressor to be vented away from the sample compartment to promote efficient cooling. Further, because the compressor is well above floor level, it is protected from the corrosive effects of heavier-than-air gases, hose spray and splatter, and the like.
The present invention also provides for flow-proportional variable sample volumes, which has heretofore been unavailable in automatic fluid sampling devices. Because fluid samples represent the source stream better when collected in proportion to the source stream flow rate, many federal and state discharge permits require that samples be collected in proportion to flow. While known automatic fluid sampling devices are capable of collecting fixed-volume samples at time intervals which vary proportionally with flow rate, they are incapable of varying the actual sample volume in proportion to flow rate. Known "variable time, constant volume" sampling devices are thus inherently incapable of providing the "constant time, variable volume" sampling called for by some discharge permits.